Intramolecular Diels-Alder reaction in enyne-allenes: a computational investigation and comparison with the Myers-Saito and Schmittel reactions

摘要

The reaction mechanisms as well as substituted effect and solvent effect of the enyne-allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers-Saito and Schmittel reactions. The MyersSaito reaction of non-substituted enyne-allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C-2-C-7 cyclization and more exothermic (Delta E-R = -69.38 kcallmol). For R-1 = CH3 and t-Bu, the increasing barrier of the C-2-C-7 cyclization is higher than that for the C-2-C-6 cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron-releasing group. Moreover, the strong steric effect of R, = t-Bu would shift the C-2-C-7 cyclization to the [4 + 2] cycloaddition. On the other hand, for R-1 = Ph, NH2, O-, NO2, and CN substituents, the barrier of the C-2-C-6 cyclization would be more diminished than the C-2-C-7 cyclization due to strong mesomeric effect; the reaction path of C2-C7 cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential-energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright